Export pytest test coverage percentage into a shield for the README

[vjeeva]

Is your feature request related to a problem? Please describe.
I just want to see a shield of the test coverage for pgbelt. The tests run in a docker container, so getting the coverage info into default GHA actions is not as trivial.

Describe the solution you'd like
I want a shield on the README saying the pytest test coverage percentage.

Describe alternatives you've considered
Not doing this, it's still nice to have.

Additional context
I'd like a shield from shields.io.